Gosh, I knew that as well! It is documented by kasparov in vol 2 of the great processors (see what I did there?) when discussing a very important save by Botvinnik against Euwe in 1946. That endgame, which Euwe thought to be analogous to the Lasker game, turned out to be slightly but crucially different. These being the days of adjournments, Botvinnik had found the difference in the quiet of his study, or something.